Are you sure you need one? When I bought my 74 Challenger with a 340 it had the adapter. It always seemed to have a very slight leak, just couldn't get it to totally seal. We took it off and have never looked back. I don't see any need for it unless the oil filter is interfering with something else.

My 318 originally did not come with that adapter. I installed one so that the oil filter would clear the cheap headers I put on at that time. Have since got rid of the headers but left the adapter on. No leaks in my case.